To insure that current is collected efficiently from amorphous silicon photovoltaic cells it is necessary to have at least three things: 1) a good current collection grid system distributed over the top conducting oxide (TCO) of the cell; 2) a good conductive link between the top conducting oxide (TCO) and the current collection grid; and 3) a means of carrying the collected current off the active area of the cell to the major bus bars and terminals.
The current collection grids used in existing manufacturing and pilot plants are fabricated by silk screening a silver paste conductive polymer compound onto the top conducting oxide. These pastes are formulated with very fine conductive powders to allow them to flow through the silk screens without clogging. However, the pastes are expensive, and shading from the unavoidably wide grids contributes to a significant portion of the total current collection power losses. In addition to the silver paste grid, a carbon paste grid is screened underneath the silver grid to act as a buffer against shorting. This shorting could occur if the silver paste were to be printed directly over a defective area that exposes a small area of the stainless steel substrate. This is an unfortunate result of the fact that the pastes contain particles small enough to seep into these pinhole defects. With carbon paste underneath the silver paste grid however, the carbon paste would coat any such defective area instead of the silver paste. Because the carbon paste has a much higher resistivity than the silver paste it lessens the effect of any inadvertent electrical connection to the stainless steel substrate through the defect.
Unfortunately, this carbon paste screening step adds to the cost and complexity of the photovoltaic module manufacturing process. It also creates the problem of achieving proper registration between the carbon and silver grid screening. To lessen the required registration precision, the carbon paste grid pattern is made somewhat larger than that of the silver pattern, but this increases shading and hence, power losses.
Another problem with the silver paste grids is that since the electrical conductivity of the grids is not nearly as high as desirable, wire bus bars need to be run across the surface of the module to connect up to the grids and carry their current away with an acceptably low electrical power loss. Present module assembly technology involves connecting the grids to the wire bus bars with dots of silver paste, further adding to the cost and complexity of their manufacture. Also, the relatively thick (26 gauge) wire bus bar, and especially the silver paste dots protrude substantially in height above the surface of the cell causing them to come undesirably close to the front surface of the lamination material in which the photovoltaic module is encapsulated. This in turn can cause them to fail critical tests such as high voltage, scratch and thermo-cycling tests, indicating that their electrical isolation is compromised.
Alternatives to conductive paste grids include evaporated or plated metal coatings enhanced with a solder coating. Although the conductivity of such grids is good, evaporating or plating fine lines of metal is difficult and costly.
The present invention uses thin (34-44, preferably 38-42 gauge) solid copper wires and conductive fibers or filaments, such as carbon fibers, as a combined current collection grid/bus bar system wherein the wires serve the dual purpose of collecting the current from the top conducting oxide as well as carrying it away from the active area of the cell. The advantages of using such wire grids in place of the silk screened silver and carbon paste grids, are 1) potential for reduced manufacturing and module costs, 2) reduced electrical power loss, 3) reduced power loss due to shading, and 4) increased reliability and defect resistance.
Known wire gridding techniques presently rely on standard conductive paste compounds in order to make a physical bond, as well as an electrically conductive link, between the wire grid and the top conductor. These conductive pastes, which are generally opaque or nearly opaque, must be applied carefully so that they do not spread excessively on the top conducting oxide and thereby increase shading, which contributes to power loss. Also, it is important that the adhesion of the conductive paste, which becomes hard upon curing, be maintained throughout the life of the module since it is necessary for good electrical contact.
Advantages over traditional wire grid current collecting systems include: